Please check the Start and End Date of the Event. The start date is the date when the event starts. When the event start date passes, it no longer shows in the 'My Events', section; however, it can still be booked until the End Date passes. Therefore, if the event's End Date is in the past compared to the current date, then the event will no longer be available for booking on the Portal.
Read more about Adding an Event to the Operate Member Portal.